Drilling fluid

ABSTRACT

A drilling fluid and method for drilling in a coal containing formation with a mixed metal-viscosified drilling fluid including at least 0.05% calcium sulfate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/348,629 filed Nov. 10, 2016 which is presently pending. U.S.application Ser. No. 15/348,629 is a divisional application of U.S. Ser.No. 13/817,635 filed May 2, 2013, now U.S. Pat. No. 9,850,416 issuedDec. 26, 2017. U.S. Ser. No. 13/817,635 is a 371 of PCT/CA2011000973filed Aug. 26, 2011, which claims benefit of U.S. 61/377,202 filed Aug.26, 2010 and to U.S. 61/417,662 filed Nov. 29, 2010.

FIELD

This invention relates to methods and fluids used for drilling andcompleting oil wells.

BACKGROUND

The process of drilling a hole in the ground for the extraction of anatural resource requires a fluid for removing the cuttings from thewellbore, lubricating and cooling the drill bit, controlling formationpressures and maintaining hole stability.

Many earth formations contain coal seams through which a wellbore mustbe drilled to either access the coal itself or reservoirs of interestbelow the coal.

For coal bed methane (CBM) wells, minimization of formation damage isparamount given the lower permeability of coal seams than conventionalreservoirs. A fluid that minimizes formation damage and reduces wholemud loss by limiting the invasion into the cleats and fractures andpermits easy flow back has been developed, termed herein as mixedmetal-viscosified drilling fluids. Such drilling fluids include a mixedmetal viscosifier, which is an inorganic particle based onmagnesium/aluminum oxides and/or hydroxides. They are commonly knownmixed metal hydroxides and sometimes referred to as mixed metal oxide(MMO), mixed metal hydroxide (MMH) and combinations of mixed metal oxideand hydroxide (MMOH). Mixed metal viscosifier is a mixed metal layeredhydroxide compound of the following empirical formula:Li_(m)D_(d)T(OH)_((m+2d+3+na))A_(a) ^(n),Where

-   -   m represents the number of Li ions present (preferably 0);    -   D represents divalent metal ions such as Mg, Ca, Ba, Sr, Mn, Fe,        Co, Ni, Cu, Zn, most preferably Mg, or mixtures thereof;    -   d is the number of ions of D in the formula, preferably from 0        to about 4, and most preferably about 1;    -   T represents trivalent metal ions and may be Al, Ga, Cr or Fe,        preferably Al;    -   A represents monovalent or polyvalent anions other than OH ions        and may be inorganic ions such as: halide, sulfate, nitrate,        phosphate, carbonate, most preferably halide, sulfate,        phosphate, or carbonate, or they may be hydrophilic organic ions        such as glycolate, lignosulfate, polycarboxylate, or        polyacrylates;    -   a is the number of ions of A in the formula;    -   n is the valence of A; and    -   (m+2d+3+na) is equal to or greater than 3.

Particularly preferred is the mixed metal hydroxide of the formulaAl/Mg(OH)_(4.7)Cl_(0.3).

Mixed metal-viscosified drilling fluids include an aqueous-based mixtureof at least one of the mixed metal moieties and an amount of bentonite.The rheology of mixed metal-viscosified drilling fluids limits fluidinvasion into the formation due to high viscosity but the main formationprotection comes from the formation of an external filter cake that iseasy to remove. Simple displacement to water or brine should besufficient for the well to flow back and remove the filter cake.

Unfortunately, however, the rheology of mixed metal-viscosified drillingfluids has broken down when coming into contact with coal finesgenerated from drilling into coal seams, especially young coal. When thedrilling fluid comes in contact with coal fines generated by drillingthrough the seams, the fluid thins, moving toward the rheology of waterand therefore loses many of its beneficial properties. Since coal seamsare, in fact, often considered loss zone formations, and are weak andfriable, the unsuitability of mixed metal-viscosified drilling fluidsfor drilling in coal containing formations is particularly problematic.

In WO 2008/106786, published Sep. 12, 2008, the present applicantproposed the use of potassium salts including, for example, one or moreof potassium sulfate, potassium chloride, potassium acetate andpotassium formate to substantially maintain the rheology of mixedmetal-viscosified drilling fluids when drilling with coal contaminants.

SUMMARY OF THE INVENTION

In accordance with a broad aspect of the present invention, there isprovided a method for drilling in a coal containing formation, themethod comprising: providing a mixed metal-viscosified drilling fluidincluding at least 0.05% (w/v) calcium sulfate; circulating the drillingfluid through the well; and drilling into the coal seam.

In accordance with another broad aspect of the present invention, thereis provided a drilling fluid comprising: an aqueous mixture of bentoniteand a mixed metal viscosifier with a pH above about pH 10; and at least0.05% (w/v) calcium sulfate.

It is to be understood that other aspects of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein various embodiments of the invention areshown and described by way of example. As will be realized, theinvention is capable for other and different embodiments and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present invention.Accordingly the detailed description and examples are to be regarded asillustrative in nature and not as restrictive.

DESCRIPTION OF VARIOUS EMBODIMENTS

The detailed description and examples set forth below are intended as adescription of various embodiments of the present invention and are notintended to represent the only embodiments contemplated by the inventor.The detailed description includes specific details for the purpose ofproviding a comprehensive understanding of the present invention.However, it will be apparent to those skilled in the art that thepresent invention may be practiced without these specific details.

Until now mixed metal-viscosified drilling fluids have been usedgenerally unsuccessfully in coal seams due to the fluid thinning effectfrom the coal. It is believed that the polyanionic nature of coal fines,such as of lignite and lignosulfonates, interfere with the electrostaticinteractions of the mixed metal moiety and the bentonite in the drillingfluid, sometimes resulting in a complete collapse of the fluid'srheology.

We have determined that some salts reduce or prevent the thinning effectfrom drilling coals with mixed metal-viscosified fluids. We previouslyproposed the use of potassium salts including, for example, one or moreof potassium sulfate, potassium chloride, potassium acetate andpotassium formate to substantially maintain the rheology of mixedmetal-viscosified drilling fluids when drilling with coal contaminants.We have now found that calcium sulfate, such as, for example, in theform of gypsum, may also substantially maintain the rheology of mixedmetal-viscosified drilling fluids when drilling with coal contaminants.Calcium sulfate prevents the thinning effect of drilling coals withmixed metal-viscosified fluids, such as those based on MMH, MMO or MMOH.Such a salt may also add a benefit of shale swelling inhibition,possibly as a result of the presence of the calcium ion from the salt.

The amount of salt added to the drilling fluid may be determined by theamount of coal to be drilled and/or by the shale reactivity. Forexample, younger coals, more so than older coals, tend to create greaterrheological instability for mixed metal-viscosified drilling fluids and,thus, higher concentrations of the salts may be useful in the drillingfluid. Also, if it is determined that there are significant coaldeposits through which the well must be drilled, again higherconcentrations of the salts may be useful.

For calcium sulfate, concentrations greater than 0.05% (weight byvolume), may be effective in the mixed metal-viscosified drilling fluid.While amounts of up to 5% or more may be used, generally concentrationsof 0.05%-1.0% (weight by volume) calcium sulfate and, for example,0.05-0.5% salt (weight by volume) or 0.1-0.5% concentrations have beenfound to be both effective for stabilizing the drilling fluid againstadverse rheological changes due to coal contamination and advantageousin terms of economics. In younger coals or where significant coaldeposits must be drilled, higher concentrations (for example greaterthan 0.3% and for example 0.3-1.0%) of calcium sulfate in the drillingfluid may be useful. It is believed that the calcium sulfate reachessaturation at about 2 to 3 kg/m3, (0.2 to 0.3% (w/v)), but excessamounts may be added without an adverse effect and in fact may create abuffer of salt to maintain activity, provided the fluid remains a liquidwhich can be circulated through the wellbore. Generally, based on acost/benefit analysis, an upper limit of 1.0% or more likely 0.5% isconsidered sound.

If desired, potassium salt may also be added to the drilling fluid. Awide range of potassium salt concentrations, such as concentrationsgreater than 1% (weight by volume), may be effective in the mixedmetal-viscosified drilling fluid. Generally concentrations of 1-10%(weight by volume) salt and, for example, 1-5% salt (weight by volume)concentrations have been found to be both effective for stabilizing thedrilling fluid against adverse rheological changes due to coalcontamination and advantageous in terms of economics. In younger coalsor where significant coal deposits must be drilled, higherconcentrations (for example greater than 3% and for example 3-10%) ofpotassium salts in the drilling fluid may be useful.

Although the salt may be added after the coal contamination occurs, itis recommended to pre-treat the system for best results. In oneembodiment, for example, the surface hole can be drilled down toapproximately the level of the first coal deposit using any drillingfluid of interest, including for example, prior art mixedmetal-viscosified drilling fluids. When it is determined that the coalseam is close below bottom hole or when the coal seam has been reached,the drilling fluid may be changed over to a drilling fluid according tothe present invention, including a mixed metal-viscosified drillingfluid containing an amount of a calcium sulfate. When consideringsurface drilling and possible contact with aquifers and surface water,calcium sulfate may be particularly of interest.

Alternately, the borehole may be drilled down to and through a coal seamusing a drilling fluid according to the present invention. For example,the entire well substantially from surface, which it will be appreciatedmay include drilling from surface or from below the overburden or afterthe casing point, may be drilled using a drilling fluid according to thepresent invention. For example, the use of the current drilling fluidmay be initiated upon initiation of the drilling operation.

After drilling through the coal seams in the path of the borehole, thepresent drilling fluid may continue to be used for the remainder of thewellbore or other drilling fluids may be used. However, if coal finesmay continue to become entrained in the drilling fluid, for examplewhere a coal seam remains open to contact by the drilling fluid, it maybe useful to continue using the present drilling fluid until drilling iscomplete or the possibility of coal contamination is eliminated. Ifdesired, the drilling fluid returning to the mud tanks at surface may bemonitored to determine the concentration of potassium salt and/orcalcium sulfate therein, as well as other parameters, to ensure thatappropriate levels and fluid characteristics are maintained. Forexample, any one or more of the bentonite, mixed metal viscosifier,base, or the salt being employed (the potassium salt and/or calciumsulfate) may be added during drilling to adjust the drilling fluidparameters. In one embodiment, for example, an amount of mixed metalviscosifier may be added to the fluid during the course of a drillingoperation where reactive formations are drilled and drill cuttingsbecome incorporated to change the rheology of the drilling fluid. Insuch a case, the addition of an amount of mixed metal viscosifier cancause the viscosity of the fluid to increase.

As will be appreciated, the drilling fluid may be circulated through thedrill string, drill bit and well bore annulus while drilling.Circulation of the drilling fluid may continue even when drilling isstopped in order to condition the well, prevent string sticking, etc.

During the drilling and circulation, the yield point of the drillingfluid may be maintained above 10 Pa to provide advantageous effects.

Mixed metal-viscosified drilling fluids include bentonite and a mixedmetal viscosifier in water and are pH controlled.

Bentonite is commonly used in drilling fluids and its use will be wellunderstood by those skilled in the art. While various forms of bentonitemay be used, bentonites that contain polyanionic additives or impuritiesshould be avoided, with consideration as to the electrostaticinteraction of the bentonite and mixed metal viscosifier. An untreatedbentonite may be particularly useful. Such an untreated bentonite mayalternately be known commercially as un-peptized or natural bentoniteand has a high content of sodium montmorillonite or Wyoming bentonite.Herein, the general term bentonite includes at least all of these forms.

As noted above, a mixed metal viscosifier is an inorganic particle basedon magnesium/aluminum oxides and/or hydroxides. While sometimes referredto as mixed metal oxide (MMO), mixed metal hydroxide (MMH) andcombinations of mixed metal oxide and hydroxide (MMOH), mixed metalviscosifiers are commonly known as mixed metal hydroxides and areunderstood to be represented by the formula:Li_(m)D_(d)T(OH)_((m+2d+3+na))A_(a) ^(n)

-   -   where        -   m represents the number of Li ions present including 0;        -   D represents divalent metal ions such as Mg, Ca, Ba, Sr, Mn,            Fe, Co, Ni, Cu and Zn;        -   d is a number from 1 to about 4;        -   T is a trivalent metal;        -   A is a mono or polyvalent anion other than OH;        -   a represents the number of ions of A ions present;        -   n is the valence of A; and        -   (m+2d+3+na) is equal to or greater than 3.

In one embodiment, the mixed metal hydroxide has the formula:D_(d)T(OH)_((2d+3+na))A_(a) ^(n)

-   -   where        -   D represents divalent metal ions such as Mg, Ca, Ba, Sr, Mn,            Fe, Co, Ni, Cu and Zn;        -   d is a number from 1 to about 4;        -   T is a trivalent metal;        -   A is a mono or polyvalent anion other than OH;        -   a represents the number of ions of A ions present;        -   n is the valence of A; and        -   (2d+3+na) is equal to or greater than 3.

For example, the divalent metal may be Mg and the trivalent metal may beAl. In one embodiment, the mixed metal viscosifier of greatest interestis a mixed metal hydroxide having the formula MgAl(OH)_(4.7)Cl_(0.3).

Mixed metal viscosifiers are commercially available such as from BASFOilfield Polymers Inc. under the trademark Polyvis II™.

Generally, mixed metal-viscosified drilling fluids may include lowconcentrations of bentonite (for example, about 15 to 50 kg/m3 or 25 to45 kg/m3 bentonite in water). Considering that many bentonite based(non-mixed metal) drilling fluids can contain many multiples more (i.e.two to four times) bentonite than in a mixed metal-viscosified drillingfluid, it can be appreciated that the viscosity generated using such lowconcentrations of bentonite for mixed metal-viscosified drilling fluidsmight be insufficient for hole cleaning. The addition of mixed metaloxide, mixed metal hydroxide or mixed metal oxide and hydroxide,including activated forms thereof, at a weight ratio of 1:8 to 1:12 or1:9.5 to 1:10.5 to the bentonite produces a stable fluid when the pH isinitially maintained above about 10.0 and possibly between about 10.5and 13, as may be achieved by addition of caustic soda and/or causticpotash. While other bases may be used to adjust the pH, care may betaken to avoid precipitation with calcium sulfate. Once thebentonite/mixed metal viscosifier reaction is complete and a gel isformed, it appears that the pH can be lowered to pH 9 or possibly evenlower without any significant loss in viscosity.

In one embodiment, a mixed metal-viscosified drilling fluid may includean aqueous mixture of 25 to 45 kg/m3 bentonite, a mixed metal moiety ina weight ratio of about 1:10 MMO, MMH or MMOH to bentonite, pHcontrolled to greater than pH 11 and 0.05 to 1.0% calcium sulfate.

If desired, an amount of potassium salt may be added.

Additives for fluid loss control, lost circulation, etc. may be added tothe drilling fluid mixture, as desired. Non or minor-ionic additives maybe most useful. Some examples may include starch for fluid lossreduction, organophillic lost circulation materials (LCM), etc. Simpletesting may verify the compatibility of any particular additive with thedrilling fluid.

To produce the drilling fluid, the bentonite may first be hydrated inwater. Then the mixed metal moiety is added and pH is adjusted. The saltcan be added to the aqueous mixture of bentonite and mixed metal anytime before it is needed for drilling with coal contamination. Additivessuch as LCM, fluid loss control agents, etc. can also be added whenappropriate, as will be appreciated.

A typical drilling fluid formulation may be according to Table 1A.

TABLE 1A A typical drilling fluid useful for drilling in coal-containingformations Product Concentration Notes Untreated bentonite 30 kg/m3Prehydrate first in fresh water MMH or MMO or 3 kg/m3 MMOH Caustic Soda0.5 to 1 kg/m3 To control pH at 11-12.5 Potassium Sulfate 20 to 50 kg/m3Starch 5 to 10 kg/m3

Another typical drilling fluid formulation may be according to Table 1B.

TABLE 1B A typical drilling fluid useful for drilling in coal-containingformations Product Concentration Notes Untreated bentonite 25 to 45kg/m3 Prehydrate first in fresh water MMH or MMO or 2.5 to 4.5 kg/m3MMOH Caustic Soda 0.5 to 1 kg/m3 To control pH at 11-12.5 CalciumSulfate 0.5 to 50 kg/m3 Starch 5 to 10 kg/m3

The following examples are included for the purposes of illustrationonly, and are not intended to limit the scope of the invention orclaims.

EXAMPLES Example I

In the following examples, drilling fluids were prepared according tothe sample descriptions by hydrating the bentonite, adding the mixedmetal moiety and adjusting the pH, as needed. Thereafter, gypsum andlignite (to simulate coal contamination) were added in various orders ofaddition to measure the effects of both agents on their own and incombination on fluid rheology.

The rheological properties have been tested using a Fann™ 35 andBrookfield™ viscometers. As will be appreciated, in the followingexamples: RPM means revolutions per minute, PV means plastic viscosity,YP means yield point, LSRV means low-shear-rate viscosity and MBT meansmethylene blue test

Federal Supreme™ is used as the bentonite (untreated). Federal Supremeis a natural untreated bentonite (sodium montmorillonite). The MMH usedis Polyvis II™ from BASF.

Below is a set of experiments with a basic 40 kg/m3 natural bentonite(untreated sodium montmorillonite) slurry that was pre-hydrated for 16hours in fresh water followed by additions of Mixed metal hydroxide(MMH; BASF Polyvis II) and then caustic to raise the pH to 11.0 orabove. The slurry quickly becomes viscous with the addition of thecaustic. The rheology is measured with a Fann 35 rotary viscometer. Theeffect of the addition of small amounts (5 g/L) of lignite to this thickslurry is measured. In the case of a control, the fluid changes fromvery viscous to very thin (almost the consistency of water) after theaddition of lignite. This is now compared to a slurry that has beenpre-treated with 5 g/L of gypsum prior to the addition of lignite to thetest slurry. It can be seen that the thinning effect of lignite iscompletely avoided.

TABLE 2 Composition of Sample #1 Products Sample #1 Untreated Bentonite40 kg/m3 MMH 4 kg/m3 Caustic 0.5 kg/m3

TABLE 3 Results without and with the addition of Gypsum Sample #1 + 5kg/m3 Mud Sample #1 + Gypsum + Properties Sample #1 5 kg/m3 Lignite 5kg/m3 Lignite 600 RPM 134 21 153 300 RPM 128 12 134 200 RPM 121 9 129100 RPM 112 6 115  6 RPM 71 1 68  3 RPM 69 1 65 PV (mPa*s) 6 9 19 YP(Pa) 61 1.5 57.5 pH 10.7 10.7 10.5

The above experiment is repeated but with a slurry containing less MMHand Natural bentonite (30 kg/m3). Additional caustic is added along withthe gypsum to maintain a constant pH in the slurry.

TABLE 4 Composition of Sample #2 Products Sample #2 Untreated Bentonite30 kg/m3 MMH 3 kg/m3 Caustic 0.5 kg/m3

TABLE 5 Results without and then with the addition of Gypsum Sample #2 +5 kg/m3 Sample #2 + Gypsum + 5 kg/m3 0.2 kg/m3 Mud Gypsum + Caustic +Properties Sample #2 0.2 kg/m3 Caustic 5 kg/m3 Lignite 600 RPM 100 11382 300 RPM 88 100 73 200 RPM 81 90 67 100 RPM 70 76 58  6 RPM 42 44 34 3 RPM 41 35 29 PV (mPa*s) 12 13 9 YP (Pa) 38 43.5 32 pH 11 11 11

In another test a MMH-bentonite (30 kg/m3) slurry is examined; first 5kg/m3 lignite is added to the basic slurry and then gypsum is addedafter the lignite: The MMH-bentonite slurry is mixed by the same methodas the experiment above; that is first the untreated bentonite is mixedand hydrated in fresh water for at least 16 hours. Then the MMH is mixedin followed by the addition of the caustic to raise the pH to 11.0 orabove to initiate the viscosifying process.

The rheology is measured using a Fann 35 rotary viscometer and recorded.Then 5 g/l of lignite is added and the rheology is measured and comparedagain. Finally 5 g/I gypsum is added to the thin mixture containinglignite and allowed to mix for 30 minutes followed by caustic to raisethe pH again to 11; when the rheology is measured again.

TABLE 6 Results without and then with the addition of Gypsum Sample #2 +5 kg/m3 Lignite Mud Sample #2 + then + 5 kg/m3 Properties Sample #2 5kg/m3 Lignite Gypsum + Caustic 600 RPM 100 8 52 300 RPM 90 4 46 200 RPM75 3 42 100 RPM 67 2 37  6 RPM 22 0 23  3 RPM 19 0 20 PV (mPa*s) 10 4 6YP (Pa) 40 0 20 pH 11.5 10.5 11

This experiment shows that the adverse thinning effect of lignite onthese slurries can be at least partially reversed with the addition ofgypsum.

Example II

In the following examples, drilling fluids were prepared according tothe sample descriptions by hydrating the bentonite, adding the mixedmetal moiety and adjusting the pH, as needed. Thereafter, any additives,including potassium salt if any, were added.

TABLE 7 Composition of Sample #3 Products Sample #3 Untreated Bentonite30 kg/m3 MMH 3 kg/m3 Caustic 0.5 kg/m3 Starch 10 kg/m3

TABLE 8 Results without the addition of Salt Mud Sample #3 + Sample #3 +Properties Sample #3 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 86 47 43300 RPM 64 29 25 200 RPM 53 21 18 100 RPM 40 13 10  6 RPM 19 2 1.5  3RPM 17 1 1 10 sec Gel (Pa) 8 1 0.5 PV (mPa*s) 22 18 18 YP (Pa) 21 5.53.5 LSRV (cP) 54,000 12,000 0 Temperature (° C.) 22.8 22.3 23.0

TABLE 9 Results using Potassium Chloride Sample #3 + Sample #3 + MudSample #3 + 2% KCl + 2% KCl + Properties 2% KCl 5 kg/m3 Lignite 15 kg/m3Lignite 600 RPM 66 47 44 300 RPM 52 31 27 200 RPM 46 23 21 100 RPM 38 1614  6 RPM 18 4 3  3 RPM 16 3 2 10 sec Gel (Pa) 7 2 1.5 PV (mPa*s) 14 1617 YP (Pa) 19 7.5 5 LSRV (cP) 25,000 12,000 9,000 Temperature (° C.)21.6 22.1 22.3

TABLE 10 Results using Potassium Acetate Sample #3 + Sample #3 + Sample#3 + 2% Pot. 2% Pot. Mud 2% Pot. Acetate + Acetate + Properties Acetate5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 66 52 48 300 RPM 47 38 35 200RPM 39 32 29 100 RPM 30 25 22  6 RPM 12 10 10  3 RPM 8 8 7 10 sec Gel(Pa) 4 4 4 PV (mPa*s) 13 14 13 YP (Pa) 20 12 11 LSRV (cP) 31,000 20,00012,000 Temperature (° C.) 23.2 23.3 23.2 Note: Lignite dissolves slower.

TABLE 11 Results using Potassium Formate Sample #3 + Sample #3 + Sample#3 + 2% Pot. 2% Pot. Mud 2% Pot. Formate + Formate + Properties Formate5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 66 47 42 300 RPM 53 32 28 200RPM 47 26 22 100 RPM 38 18 16  6 RPM 19 6 5  3 RPM 18 4 4 10 sec Gel(Pa) 7 2 2 PV (mPa*s) 13 15 14 YP (Pa) 20 8.5 7 LSRV (cP) 21,000 13,00012,000 Temperature (° C.) 22.1 22.3 22.6

TABLE 12 Results using Calcium Nitrate Sample #3 + Sample #3 + Sample#3 + 2% Calcium 2% Calcium Mud 2% Calcium Nitrate + Nitrate + PropertiesNitrate 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 60 57 47 300 RPM 46 4234 200 RPM 38 34 28 100 RPM 31 27 22  6 RPM 12 11 7  3 RPM 9 9 5 10 secGel (Pa) 5 5 3 PV (mPa*s) 14 15 13 YP (Pa) 16 13.5 10.5 LSRV (cP) 33,00023,000 22,000 Temperature (° C.) 21.5 22.1 22.7 Note: Lignite dissolvesslower.

TABLE 13 Results using Calcium Chloride Sample #3 + Sample #3 + Sample#3 + 2% Calcium 2% Calcium Mud 2% Calcium Chloride + Chloride +Properties Chloride 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 61 51 47300 RPM 44 35 34 200 RPM 36 30 29 100 RPM 27 22 23  6 RPM 10 8 8  3 RPM8 7 6 10 sec Gel (Pa) 3.5 3.5 3 PV (mPa*s) 17 16 13 YP (Pa) 13.5 9.510.5 LSRV (cP) 27,000 23,000 22,000 Temperature (° C.) 24.4 24.4 24.2Note: Lignite dissolves slower.

TABLE 14 Results using Potassium Sulfate Sample #3 + Sample #3 + Sample#3 + 2% Pot. 2% Pot. Mud 2% Pot. Sulfate + Sulfate + Properties Sulfate5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 75 42 34 300 RPM 60 29 21 200RPM 52 24 16 100 RPM 41 18 11  6 RPM 21 8 2.5  3 RPM 19 7 2 10 sec Gel(Pa) 9 4 2.5 PV (mPa*s) 15 13 13 YP (Pa) 22.5 8 4 LSRV (cP) 32,00030,000 25,000 Temperature (° C.) 24.4 24.0 21.3

TABLE 15 Results using Potassium Chloride Sample #1 + Sample #1 + MudSample #1 + 5% KCl + 5% KCl + Properties 5% KCl 5 kg/m3 Lignite 15 kg/m3Lignite 600 RPM 61 52 46 300 RPM 49 39 35 200 RPM 45 35 32 100 RPM 42 3230  6 RPM 16 15 15  3 RPM 12 11 10 10 sec Gel (Pa) 6 6 5 PV (mPa*s) 1213 11 YP (Pa) 18.5 13 12 LSRV (cP) 30,000 18,000 21,000 Temperature (°C.) 20.1 20.1 20.1

TABLE 16 Results using Potassium Acetate Sample #3 + Sample #3 + Sample#3 + 5% Pot. 5% Pot. Mud 5% Pot. Acetate + Acetate + Properties Acetate5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 63 48 44 300 RPM 55 37 36 200RPM 51 36 34 100 RPM 47 34 32  6 RPM 14 20 16  3 RPM 9 11 11 10 sec Gel(Pa) 5 5 6 PV (mPa*s) 8 11 8 YP (Pa) 23.5 13 14 LSRV (cP) 27,000 14,00033,000 Temperature (° C.) 20.1 20.1 20.1 Note: Lignite dissolves slower.

TABLE 17 Results using Potassium Formate Sample #1 + Sample #1 + Sample#1 + 5% Pot. 5% Pot. Mud 5% Pot. Formate + Formate + Properties Formate5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 50 46 42 300 RPM 40 33 33 200RPM 37 30 30 100 RPM 32 28 29  6 RPM 9 9 14  3 RPM 5 8 10 10 sec Gel(Pa) 3 4 5 PV (mPa*s) 10 13 9 YP (Pa) 15 10 12 LSRV (cP) 30,000 29,00031,000 Temperature (° C.) 20.1 20.1 20.1

TABLE 18 Results using Calcium Nitrate Sample #3 + Sample #3 + Sample#3 + 5% Calcium 5% Calcium Mud 5% Calcium Nitrate + Nitrate + PropertiesNitrate 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 58 49 44 300 RPM 52 4238 200 RPM 50 41 37 100 RPM 47 35 32  6 RPM 12 11 14  3 RPM 8 8 8 10 secGel (Pa) 5 4.5 4.5 PV (mPa*s) 6 7 6 YP (Pa) 23 17.5 16 LSRV (cP) 35,00043,000 23,000 Temperature (° C.) 20.1 20.1 20.1 Note: Lignite dissolvesslower.

TABLE 19 Results using Calcium Chloride Sample #3 + Sample #3 + Sample#3 + 5% Calcium 5% Calcium Mud 5% Calcium Chloride + Chloride +Properties Chloride 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 63 48 43300 RPM 50 37 34 200 RPM 42 34 31 100 RPM 35 29 29  6 RPM 13 12 13  3RPM 10 9 11 10 sec Gel (Pa) 6.5 6.5 7 PV (mPa*s) 13 11 9 YP (Pa) 18.5 1311.5 LSRV (cP) 40,000 37,000 27,000 Temperature (° C.) 20.1 20.1 20.1Note: Lignite dissolves slower.

TABLE 20 Results using Potassium Sulfate Sample #3 + Sample #3 + Sample#3 + 5% Pot. 5% Pot. Mud 5% Pot. Sulfate + Sulfate + Properties Sulfate5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 165 128 91 300 RPM 150 115 76200 RPM 143 109 71 100 RPM 131 100 63  6 RPM  85 67 42  3 RPM  37 58 3910 sec Gel (Pa)  16 29 22 PV (mPa*s)  15 13 15 YP (Pa)   77.5 51 30.5LSRV (cP) 100,000+   80,000 67,000 Temperature (° C.)   20.1 20.1 20.1

TABLE 21 Results using Sodium Sulfate Sample #3 + Sample #3 + Sample#3 + 2% Sodium 2% Sodium Mud 2% Sodium Sulfate + Sulfate + PropertiesSulfate 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 179 39 31 300 RPM 15525 19 200 RPM 143 20 15 100 RPM 123 14 9  6 RPM 72 8 3  3 RPM 63 7 2 10sec Gel (Pa) 31 5 2.5 PV (mPa*s) 24 14 13 YP (Pa) 65.5 5.5 4 LSRV (cP)90,000 50,000 28,000 Temperature (° C.) 22.0 22.0 22.0

TABLE 22 Results using Sodium Sulfate Sample #3 + Sample #3 + Sample#3 + 5% Sodium 5% Sodium Mud 5% Sodium Sulfate + Sulfate + PropertiesSulfate 5 kg/m3 Lignite 15 kg/m3 Lignite 600 RPM 207 48 33 300 RPM 17438 22 200 RPM 152 35 18 100 RPM 124 31 13  6 RPM 74 27 11  3 RPM 67 2610 10 sec Gel (Pa) 28 14 9 PV (mPa*s) 33 10 11 YP (Pa) 70.5 14 5.5 LSRV(cP) 100,000 100,000 80,000 Temperature (° C.) 22.0 22.0 22.0

Example III

Background: Nr Wetaskiwin, Alberta, Drilled 222 mm hole to IntermediateCasing Depth of 1425mMD and set casing at ˜86.2 degrees inclination inthe Rex Coal formation. Set and cement 177.8 mm casing.

Drilling Fluid: 60m3 of mud is premixed with the following formulation:30 kg/m3 of natural bentonite is pre-hydrated in fresh water for 16hours. 3 kg/m3 of PolyVis II (MMH) is added over 2 hours. pH is raisedto 12.0 with caustic via chemical barrel over pre-mix tank. Fluidbecomes viscous. 50 kg/m3 of potassium sulfate is added.

Drilling in Coal: Intermediate casing shoe and cement are drilled outwith a 156 mm bit using water and then water is displaced over to thepre-mixed system, described above. This well was drilled horizontally inthe Rex Coal formation using the pre-mixed system.

Fluid Properties prior to drilling coal:

-   Premix: 60m3 circulating system.-   Depth: 1425 m (87.2 degrees inclination)-   Funnel Viscosity: 55 s/L-   Mud density: 1050 kg/m3-   pH: 12.0-   600 reading: 64-   300 reading: 61-   200 reading: 60-   100 reading: 56-   6 reading: 36-   3 reading: 23-   PV (mPa·s): 3-   YP (Pa): 29-   Gels (Pa): 11/11-   Filtrate (Fluid Loss, mls/30 min): no control-   MBT: 30 Kg/m3-   Potassium ion (mg/L): 25,000

Fluid properties after drilling to 1451 m in Rex Coal formation:

-   Depth: 1451 m (88 degrees inclination)-   Funnel Viscosity: 66 s/L-   Mud density: 1060 kg/m3-   pH: 11.5-   600 reading: 62-   300 reading: 55-   200 reading:—-   100 reading:—-   6 reading:—-   3 reading:—-   PV (mPa·s): 7-   YP (Pa): 24-   Gels (Pa): 6/10-   Filtrate (Fluid Loss, mls/30 min): 60-   MBT: 24 Kg/m3-   Potassium ion (mg/L): 22,000

It was determined that the fluid viscosity remained substantially stabledespite drilling pure coal.

Thereafter drilling continued to 1845 m in Rex Coal formation with theaddition of 15×22.7 kg sacks of non-ionic starch (Unitrol Starch) forfluid loss control into 80m3 system:

Fluid properties at depth 1845 m (91.4 degrees inclination):

-   Funnel Viscosity: 59 s/L-   Mud density: 1050 kg/m3-   pH: 12.0-   600 reading: 64-   300 reading: 56-   200 reading:—-   100 reading:—-   6 reading:—-   3 reading:—-   PV (mPa·s): 8-   YP (Pa): 24-   Gels (Pa): 9/11-   Filtrate (Fluid Loss, mls/30 min): 19-   MBT: 22 Kg/m3-   Potassium ion (mg/L): 20,400

The addition of starch doesn't affect the rheology substantially.

After drilling to 2050 m in the Rex Coal formation the fluid propertieswere as follows (89m3 system):

Depth: 2050 m (87.8 degrees inclination)

-   Funnel Viscosity: 85 s/L-   Mud density: 1050 kg/m3-   pH: 12.0-   600 reading: 80-   300 reading: 70-   200 reading: 65-   100 reading: 60-   6 reading: 47-   3 reading: 44-   PV (mPa·s): 10-   YP (Pa): 30-   Gels (Pa): 17/18-   Filtrate (Fluid Loss, mls/30 min): 15-   MBT: 25 Kg/m3-   Potassium ion (mg/L): 22,500

It was determined that a mixed metal viscosified—natural bentonite typerheology can be maintained when drilling through coal with potassiumsulfate as an additive.

Example IV

TABLE 23 Composition of Sample #4 Products Sample #4 Untreated bentonite30 kg/m3 MMH  3 kg/m3

In the following examples, drilling fluids were prepared according tothe sample descriptions and in a similar manner to Example I but withless calcium sulfate (gypsum). Sample #4 is prepared as follows: Thebentonite (Federal Supreme) is prehydrated 3 hours and then MMH (PolyvisII) is added.

Caustic soda (NaOH) is added to adjust pH, followed by gypsum and thenlignite to simulate the addition of coal.

TABLE 24 Results using calcium sulfate in bentonite - MMH solutionSample #4 + Sample #4 + Caustic + Mud Sample Sample #4 + Caustic + 2kg/m3 Gyp + Property #4 Caustic 2 kg/m3 Gyp 5 kg/m3 Lignite 600 RPM 9195 98 93 300 RPM 80 80 91 85 200 RPM 74 76 89 78 100 RPM 66 69 81 74  6RPM 42 22 24 25  3 RPM 22 17 18 18 PV 11 15 7 8 (mPa*s) YP (Pa) 34.532.5 42 38.5 pH 9.2 10.7 10.7 10.0 Sample #3 + Sample #3 + Caustic + MudCaustic + 5 kg/m3 Gyp + Property 5 kg/m3 Gyp 5 kg/m3 Lignite 600 RPM 8271 300 RPM 72 66 200 RPM 68 60 100 RPM 60 53  6 RPM 17 17  3 RPM 14 12PV 10 5 (mPa*s) YP (Pa) 31 30.5 pH 10.7 9.8

Calcium sulfate acts as a good anionic suppressant of the reactionbetween coals (lignite) and bentonite—MMH/MMO complexes. The resultingfluid retains the main characteristics—high low end rheology and shearthinning behavior.

Example V

TABLE 25 Composition of Sample #5 Products Sample #5 Untreated bentonite30 kg/m3 MMH 3 kg/m3 Caustic 0.5 kg/m3

Federal Supreme™ is used as the bentonite. The MMH is Polyvis II™.

The bentonite was prehydrated for three hours before the MMH was added.Caustic soda was added to adjust the pH.

To again investigate the effect of adding calcium sulfate to a mixedmetal viscosified fluid, gypsum (Gyp) and lignite was added to sample#5.

TABLE 26 Results using calcium sulfate in bentonite - MMH solutionSample #5 + Sample #5 + Sample #5 + Mud Sample 20 kg/m3 40 kg/m3 40kg/m3 Gyp + Property #5 Gyp Gyp 5 kg/m3 Lignite 600 RPM 71 56 54 47 300RPM 60 48 46 40  6 RPM 27 24 22 20 PV 11 8 8 7 (mPa*s) YP (Pa) 24.5 2019 16.5

Example VI

The experiment of Example V is repeated, adding commercial drillingfluid starch (M-I's Unitrol™) for fluid loss control to basebentonite—MMH solution (Sample #5). Then add gypsum and thereafterlignite.

TABLE 27 Results using calcium sulfate and starch in bentonite - MMHsolution Sample #5 + Sample #5 + Sample #5 + 6 kg/m3 Unitrol + MudSample 6 kg/m3 6 kg/m3 Unitrol + 40 kg/m3 Gyp + Property #5 Unitrol 40kg/m3 Gyp 5 kg/m3 Lignite 600 RPM 67 57 61 59 300 RPM 51 42 47 43  6 RPM13 13 17 16 PV 16 15 14 16 (mPa*s) YP (Pa) 17.5 13.5 16.5 13.5

The addition of lignite did not significantly reduce the viscosity ofthe drilling fluid.

Example VII

In the following examples, drilling fluids were prepared according tothe composition of Sample #6, with any noted additives. The bentonite(Federal Supreme) is hydrated, the mixed metal moiety (Polyvis II) addedand the pH adjusted with caustic soda. Thereafter, any other additiveswere added.

To simulate coal contamination, lignite was added.

The rheological properties have been tested using a Fann 35 andBrookfield viscometers.

TABLE 28 Composition of Sample #6 Products Sample #6 Untreated Bentonite30 kg/m3 MMH 3 kg/m3 Caustic 0.5 kg/m3

TABLE 29 Results using calcium sulfate and/or potassium sulfate inbentonite - MMH solution (gel prehydrated 16 hours) Sample #6 + Sample#6 + Sample #6 + Mud Sample 20 kg/m3 40 kg/m3 40 kg/m3 Gyp + Property #6Gyp Gyp 5 kg/m3 Lignite 600 RPM 71 56 54 47 300 RPM 60 48 46 40 200 RPM56 46 44 38 100 RPM 50 40 38 33  6 RPM 27 24 22 20  3 RPM 16 13 13 12 PV11 8 8 7 (mPa*s) YP (Pa) 24.5 20 19 16.5 Sample #6 + Sample #6 + Sample#6 + 50 kg/m3 50 kg/m3 50 kg/m3 Sample #6 + K2SO4 + K2SO4 + K2SO4 + Mud50 kg/m3 20 kg/m3 40 kg/m3 40 kg/m3 Gyp + Property K2SO4 Gyp Gyp 5 kg/m3Lignite 600 RPM 91 61 46 44 300 RPM 76 53 43 37 200 RPM 67 51 41 34 100RPM 56 46 39 30  6 RPM 21 27 23 20  3 RPM 13 20 21 18 PV 15 8 3 7(mPa*s) YP (Pa) 30.5 22.5 20 15

TABLE 30 Results using calcium sulfate, starch, calcium carbonate and/orother additives in bentonite (gel prehydrated 1 hour) Sample #6 + MudSample #6 + 6 kg/m3 Unitrol Property Sample #6 6 kg/m3 Unitrol 20 kg/m3Gyp 600 RPM 90 86 55 300 RPM 80 70 41 200 RPM 74 62 34 100 RPM 66 49 26 6 RPM 23 29 13  3 RPM 17 28 12 PV (mPa*s) 10 16 14 YP (Pa) 35 27 13.5Fluid Loss 50 10.5 18.0 (mL/ 30 min.) Sample #6 + Sample #6 + 6 kg/m3Unitrol 6 kg/m3 Unitrol + 20 kg/m3 Gyp + Mud 120 kg/m3 90 kg/m3 Cal 90kg/m3 Cal Property Natural Gel Carb 325 Carb 325 600 RPM 271 58 52 300RPM 190 54 38 200 RPM 125 49 31 100 RPM 110 38 24  6 RPM 39 19 11  3 RPM36 11 10 PV (mPa*s) 81 4 14 YP (Pa) 54.5 25 12 Fluid Loss 54 14.0 16.0(mL/ 30 min.) Sample #6 + Sample #6 + Sample #6 + 6 kg/m3 Unitrol + 6kg/m3 Unitrol + 6 kg/m3 Unitrol + Mud 20 kg/m3 Gyp + 20 kg/m3 Gyp + 20kg/m3 Gyp + Property 3% KlaStop 1.5% Shure Shale 2% Inhibidrill 600 RPM40 41 20 300 RPM 33 30 13 200 RPM 31 25 11 100 RPM 29 21 9  6 RPM 16 5 3 3 RPM 14 4 2 PV (mPa*s) 7 11 7 YP (Pa) 13 9.5 3 Fluid Loss 50 32 54(mL/ 30 min.) Sample #6 + Mud Sample Sample #6 + 40 kg/m3 Gyp + Property#6 40 kg/m3 Gyp+ 0.5 kg/m3 Caustic 600 RPM 27 16 45 300 RPM 19 12 39 200RPM 17 8 36 100 RPM 16 6 31  6 RPM 10 3 20  3 RPM 9 2 14 PV 8 4 6(mPa*s) YP (Pa) 5.5 4 16.5 pH 10.2 9.6 11.1 Mud Sample #6 + Sample #6 +Sample #6 + Property 1 kg/m3 Caustic Soda 20 kg/m3 Gyp 40 kg/m3 Gyp 600RPM 92 91 84 300 RPM 81 81 75 200 RPM 77 77 71 100 RPM 70 69 64  6 RPM46 30 28  3 RPM 43 21 21 PV 11 10 9 (mPa*s) YP (Pa) 35 35.5 33 pH 11.511.3 11.3

As is known, care may be taken in the use of some additives. As someadditives such as amines, for example amine shale inhibitors, appear todestroy the bentonite complexes with or without the presence of calciumsulfate.

Example VIII

In the following examples, drilling fluids were prepared according tothe composition of Sample #7 and some noted additives. The bentonite(Federal Supreme) is hydrated for three hours, the mixed metal moiety(Polyvis II) added and the pH adjusted with caustic soda. Thereafter,any additives were added.

To simulate coal contamination, lignite was added.

TABLE 31 Composition of Sample #7 Products Sample #7 Untreated Bentonite30 kg/m3 MMH  3 kg/m3

TABLE 32 Results using calcium sulfate in bentonite - MMH solution MudSample Sample #7 + Sample #7 + Property #7 0.01 kg/m3 Caustic 0.04 kg/m3Caustic 600 RPM 91 100 124 300 RPM 80 88 107 200 RPM 74 83 98 100 RPM 6676 86  6 RPM 42 25 28  3 RPM 22 18 20 PV 11 12 17 (mPa*s) YP (Pa) 34.538 45 pH 9.2 10.3 10.8 Sample #7 + Sample #7 + 0.01 kg/m3 0.04 kg/m3 MudSample #7 + Caustic + Caustic + Property 20 kg/m3 Gyp 20 kg/m3 Gyp 20kg/m3 Gyp 600 RPM 61 73 106 300 RPM 53 63 96 200 RPM 48 59 88 100 RPM 4153 78  6 RPM 23 16 26  3 RPM 11 13 21 PV 8 10 10 (mPa*s) YP (Pa) 22.526.5 43 pH 8.8 10.2 10.8 Sample #7 + Mud Sample #7 + Sample #7 + 2 kg/m3Gyp + Property Caustic 2 kg/m3 Gyp 5 kg/m3 Lignite 600 RPM 95 98 93 300RPM 80 91 85 200 RPM 76 89 78 100 RPM 69 81 74  6 RPM 22 24 25  3 RPM 1718 18 PV 15 7 8 (mPa*s) YP (Pa) 32.5 42 38.5 pH 10.7 10.7 10.0 Sample#7 + Sample #7 + Caustic + Mud Caustic + 5 kg/m3 Gyp + Property 5 kg/m3Gyp 5 kg/m3 Lignite 600 RPM 82 71 300 RPM 72 66 200 RPM 68 60 100 RPM 6053  6 RPM 17 17  3 RPM 14 12 PV 10 5 (mPa*s) YP (Pa) 31 30.5 pH 10.7 9.8

Gypsum slightly reduces the rheology of a bentonite-MMH fluid. Thehigher the initial pH, the lower the viscosity drop after addition ofgypsum. Adding caustic soda to raise the pH above 11 restores fluidrheology. Gypsum appears to act as a good anionic suppressant. Whenadditional shale inhibitors are added, the viscosity drops. The fluidretains the main characteristics—high low end rheology and shearthinning behavior.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are known or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 U.S.C. 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

I claim:
 1. A drilling fluid consisting essentially of: an aqueous mixture of 15 to 50 kg/m3 bentonite and a mixed metal viscosifier at a weight ratio of 1:8 to 1:12 viscosifier to bentonite, the mixed metal viscosifier being of the following empirical formula: Li_(m)D_(d)T(OH)_((m+2d+3+na))A_(a) ^(n), where: m represents the number of Li ions present; D represents divalent metal ions selected from Mg, Ca, Ba, Sr, Mn, Fe, Co, Ni, Cu, Zn or mixtures thereof; d is the number of ions of D in the formula and is from 0 to 4; T represents trivalent metal ions selected from Al, Ga, Cr or Fe; A represents monovalent or polyvalent anions selected from: halide, sulfate, nitrate, phosphate, carbonate or hydrophilic organic ions selected from alveolate, lignosulfate, polvcarboxvlate, or polvacrylates; a is the number of ions of A in the formula; n is the valence of A; and (m+2d+3+na) is equal to or greater than 3; a base to maintain a pH of the mixture above about 10; and 0.1 to 1.0% calcium sulfate (w/v), the drilling fluid having a yield point of greater than 10Pa.
 2. The drilling fluid of claim 1 wherein the drilling fluid comprises 25 to 45 kg/m3 bentonite, the mixed metal viscosifier at a weight ratio of 1:9.5 to 1:10.5 viscosifier to bentonite, and the base maintaining the pH between about 10.5 to
 13. 3. The drilling fluid of claim 1 wherein the drilling fluid comprises about 30 to 40 kg/m3 bentonite, the mixed metal viscosifier in a quantity of about 1:10 mixed metal viscosifier to bentonite with the pH controlled to greater than 11 and 0.1 to 0.5% calcium sulfate.
 4. The drilling fluid of claim 1 wherein the drilling fluid is prepared by: mixing the bentonite in water to form a bentonite mixture; adding the mixed metal viscosifier to the bentonite mixture; adjusting the pH to greater than about 10; and adding the calcium sulfate.
 5. The drilling fluid of claim 1 further comprising at least one of a fluid loss control additive and/or a lost circulation material.
 6. The drilling fluid of claim 1 wherein the calcium sulfate is in the form of gypsum.
 7. The drilling fluid of claim 1 further comprising an amount of potassium salt selected from the group consisting of potassium sulfate, potassium chloride, potassium acetate and potassium formate.
 8. The drilling fluid of claim 1 wherein the pH is adjusted using caustic soda or caustic potash.
 9. The drilling fluid of claim 1 wherein the bentonite is in the form of untreated bentonite. 